The origin and nature of Dark Matter is a question of great importance for cosmology and particle physics. The Standard Model (SM) of Particle Physics cannot provide a viable candidate and while several theoretical models have been proposed, none have been confirmed experimentally. Among these models, a very interesting one is the extension of the Standard Model with a hidden sector. A particular example is the Mirror Sector, which consists of a complete set of mirror particles with the same properties (e.g. mass and charge) as ordinary particles, but opposite chirality, i.e. mirror particles experience V+A weak interaction. Mirror particles are massive and stable and interact with ordinary matter primarily through gravitation, thus they are a very good candidate for Dark Matter.
Besides gravity, kinetic mixing of neutral particles and their mirror partners might also occur, e.g. photons, neutrinos or the neutral Higgs boson. In particular, if photons and mirror photons can mix with a coupling constant, then oscillations of the electron - positron bound state (positronium) might happen through the one-photon virtual annihilation of the triplet state. The resulting mirror partner of positronium would decay into mirror photons, and therefore the experimental signature would be the absence of the 2mc² energy expected for the positronium Standard Model decay.
The goal of this thesis is to search for such invisible decays of ortho-positronium confined in a vacuum cavity, which could be explained within the framework of the Mirror Sector. The design, construction, commissioning and first results of this search are presented. The experiment consists of a pulsed positron beam with a high efficiency tagging system, a positronium formation cavity and a calorimeter with low energy threshold and high hermeticity. The ultimate goal is to reach an experimental sensitivity to test the mixing strength at the level of 10⁻⁹, which is one order of magnitude better than the current Big Bang Nucleosynthesis limit and in a region of parameter space of great theoretical and phenomenological interest. The preliminary results set a limit on the coupling constant at the level of 3E-7, which improves the existing limit by a factor 6. This was the first search of this kind with o-Ps confined in a vacuum cavity and is thus a proof of principle that this technique is working properly. The ongoing improvement in the setup which will allow to reach the final sensitivity is also presentedShow more